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Self-adjustment takes place in two steps when the vehicle is driven in reverse and stopped with the brakes. Self-adjustment does not take place when the vehicle is backed up and allowed to roll to a stop.
The components of the self-adjustment system are shown in the illustration. The self-adjusting lever does the work. It engages the teeth of the star wheel. A cable is attached to the lever. This cable passes around a guide on the secondary (rear/main) shoe and is attached to the anchor pin. A return spring is also attached to the lever. The other end of the return spring is attached to the primary (front/small) shoe.
When the brakes are applied while traveling in reverse, the friction forces drag the brake shoes in the direction of rotation. This action is similar to the action in normal braking, only in reverse. The movement of the secondary shoe puts tension on the cable and rotates the adjusting lever upward. If the wear on the linings is sufficient to need adjustment, the lever will rise above & then rest on the next tooth up on the star wheel.
When braking is completed, the shoes move back to their normal position. As this happens, the tension on the cable is relaxed. This allows the return spring to force the lever downward, turning the star wheel as it does. Turning the star wheel activates a screw mechanism in the adjusting pin -- lengthening the pin. As the pin lengthens, the linings of both shoes are moved closer to the drum, compensating for wear. This reduces the distance that the shoes & lever move, preventing further adjustment until additional wear occurs.
This simple, mechanical system works well unless the screw mechanism in the adjusting pin has become corroded. If this is the case, the pin may not turn properly and adjustment may not take place. In this case, the cable will be found slack during the next brake service. Also remember that some drivers rarely back up. In this case, sufficient adjustment may not take place.
If the shoes are allowed to wear into the drums, a lip may remain at the inboard edge of the drum which prevents the drum from sliding off the shoes. In this case, the autoadjuster must be retracted manually using 2 tools working through the slot in the backing plate. One tool holds the lever away from the star wheel; the other turns the star wheel to shorten the autoadjuster assembly until the drum can clear the shoes.
The shoes may also wear into the backing plates, preventing them from moving away from their normal positions. This may be particularly noticeable when backing (or using the e-brake when the front is uphill) because the shoes energize the opposite direction. If the wear is shallow, it may be ground down. If it's more than fingernail-depth, the plate should be replaced, though some people weld them up and grind them back down.